Surface treatments for articles made from heat resisting alloys



SURFACE TREATMENTS FOR ARTICLES MADE FROM HEAT REISTING ALLOYSGeirionydd Llewelyn and Raymond George Ubank, Bristol, England,assignors to Bristol Acre-Engines Limited, Bristol, England, a companyof Great Britain No Drawing. Application April 1, 1957 Serial No.649,604

Claims priority, application Great Britain April 6, 1956 9 Claims. (Cl.148-'13.1)

This invention relates to the surface treatment of articles having afinished surface and made. from metals or alloys which are subject tooxidation penetrating between the grain boundaries, and moreparticularly, articles made from nickel base and cobalt base alloyscontaining chromium, the treatment being for the purpose of decreasingthe adverse effect which oxidation has upon the service life of sucharticles and of improving the resistance of such articles to thermalshock.

It is known that when such an article is used at high temperatures thesurface of the articles is subject to a type of oxidation which not onlyforms a surface layer but also penetrates between the grain boundaries.These grain boundary penetrations are particularly objectionable since.they can form the starting point for the propagation of cracks due tothermalshock, fatigue and creep, and this seriously reduces the servicelife of the article.

According to the invention there is provided a surface treatment for anarticle having a finished surface and made from a metal or alloy whichis subject to oxidation penetrating between the grain boundaries, saidtreatment comprising heating the article in contactwitha powdered alloycontaining not less than 15% by weight of titanium and not less than 6%by weight of aluminum and in an atmosphere which is inert in relation tothe metals of which the article and the powdered alloy are composed, thearticle being heated at a temperature above the normal maximum workingtemperatureof the article but not so high as to permanently adverselyaffect.

According to a feature of the inventionithe article may. be heated incontact with a powdered alloy known com-;

mercially as ferro-titanium hardener. Alternatively, however, a powderedalloy known commercially as nickeltitanium hardener may be used.

Perm-titanium hardener and nickel-titanium hardener both contain asignificant content of aluminum, the'presence of which element is, ofcourse, essential for carrying out the treatment according to theinvention as defined in the third paragraph of this specification.

As will readily be understood, the so-called hardeners referred to aboveare alloys which are rich in certain improvement metals or substancessuch as iron, nickel, titanium, aluminum, manganeseand silicon, thehardeners usually also containing certain other elements in the form ofminor traces or impurities. Such hardeners are usually used for thepurpose of adding the improvement metals or substances to other metalsor alloys, the hardeners being mixed therewith. The hardeners areusually supplied in lump form but are brittle and easily crushed Nimonictype.

to a powder. In carrying out the surface treatment according to thepresent invention the powdered alloy used should have afineness betweenand 16 mesh per inch and preferably the powdered alloy has a fineness of100 mesh per inch. e

The preferred way of carrying out a surface treatment according to theinvention is to pack the article to be treated, together with thepowdered alloy, in a box made from a heat resisting material such asstainless steel or a nickel b'ase alloy of the Nimonic type, the boxbeing provided with an outlet and with a pipe for the supply into'thebox of a gas to form an inert atmosphere which is maintained within thebox throughout the treatment by passing the gas into the box andallowing it to escape through the outlet. An inert atmosphere of argonhas been found satisfactory for carrying out the treatment according tothe invention but preferably commercially pure hydrogen, from which all,or substantially all, traces of oxygen and moisture have been removed,is used.

It is to be understood that the atmosphere should be inert in the sensethat the gas used for the atmosphere is inert in relation to the metalsof which the article and the powderedalloy are composed. The gas used toform an atmosphere may however, and preferably has, as in the case ofhydrogen, a reducing action on oxides of the metal or metals of whichthe article is composed.

Where commercially pure hydrogen is used the gas is passed overpalladised asbestos at room temperature, and then through activatedalumina, before being fed into the box. As is well known the palladisedasbestos acts as a catalyst for the combination of oxygen and hydrogentoform water and the water so formed, together with any other moisturepresent in the commercially pure hydrogen, is absorbed by the activatedalumina. The resulting-hydrogen is thus free, or substantially free,from oxygen and moisture.

It may here be noted that it is possible to carry out the surfacetreatment according to the invention by passing commercially purehydrogen into the box without first removing the oxygen and moisturefrom the gas, provided that asufficient thickness of the powdered alloyis placed around the articlebeing treated to absorb the deleteriouscomponents of the gas before the gas reaches the immediate vicinity ofthe article. Such a method of working, however, rapidly renders thepowdered alloy useless for the treatment .of further articles andconsequently the method is uneconomic.

While maintaining the flow of hydrogen through the box, the box .isheated to a temperature somewhat below the lowest temperature whichwould permanently adversely affect the physical. properties of thearticle being treated, but higher than the maximum temperature of thearticle at which the article will normally be required to work. Wherethe article is made of an alloy which would normally be subjected to asolution heat treatment, the box is heated to a temperature equal to orapproximating to the temperature to which the alloy of which the articleis made would normally be subjected in the solution heat treatment. Inthe present case it is assumed that the article being treated is made ofa nickel base alloy of the A solution heat treatment for such an alloyinvolves heating the alloy at a temperature between 1000 C. and 1190 C.and in the present case the box is heated to a temperature within thisrange.

The heating is continued for a suflicient time to form on the article amodified layer of suitable thickness. For

most purposes a layer from 0.0005 inch to 0.001 inch in thickness issuitable. In order to form a modified layer Within this range ofthickness in the present case, it is necessary to heat the box at atemperature between 1000 C. and 1190 C. for a period of time betweenforty minutes and one hour. The box is then cooled in air while stillmaintaining the atmosphere of hydrogen in the box and when cooled thearticle is removed.

It is to be understood'that the modified layer formed on the article bythe treatment just described is an original surface layer of the articlewhich has been modified by the treatment and not a surface layer whichhas subsequently been applied to the article duringthe treatment.

Instead of cooling the box before removing the article, the article maybe taken from the box while still hot and cleaned of powdered alloybefore being cooled. It is to be understood, however, that in this casethe cooling must be elfected in a way appropriate to the material fromwhich the article is made.

Instead of packing the article in a box with the powdered. alloy, aremovable coating of the powdered alloy may be applied to the surface ofthe article in any appropriate manner, as for example by mixing thepowder with a bonding medium which will give to the coating sufficientcoherence to enable it to remain in place during the heating of thearticle. The article is then heated in an inert atmosphere for apredetermined time at a predetermined temperature suificient to form onthe article a modified layer of desired thickness.

Following a surface treatment according to the invention as so fardescribed, in order to develop desired mechanical properties of thematerial forming the article, the article may require to be aged at atemperature and for a period of time appropriate to the materialcomposing the article. Thus, for example, for development of fullmechanical strength an article made from a nickel base Nimonic alloywould be aged by heating it. at be-.

tween 690 C. and 710 C. for twelve to sixteen hours and cooling in air.For some purposes however it may be desired to have the alloy in anun-aged or partially aged condition, and in this case the period of theageing heat treatment would be appropriately reduced or it would beomitted altogether.

The surface treatment according to the invention is applicable toarticles made of mild steel and other ferrous alloys subject tointergranular oxide penetration, to titanium and molybdenum base alloys,to nickel base alloys of the Nimonic type and to cobalt base heatresisting alloys, a typical example of which comprises by weight, 0.5%carbon, 28.0% chromium, 7.5% tungsten, 12.0% nickel and the remaindercobalt.

Commercial ferro-titanium hardener varies in composition as betweendifferent samples but a particular sample which proved satisfactory forsurface treating articles according to the present invention, whicharticles were made from Nimonic 90 heat resisting alloy, was composed,by weight, of 33.6% titanium, 1.4% manganese, 1.2% silicon, 16.9%aluminium and the remainder .irdn.

A sample of nickel titanium hardener which also proved satisfactory wascomposed by Weight, of 43.4% titanium, 3.7% manganese, 3.3% silicon,9.3% aluminium, 0.36% iron, 0.51% cobalt and the remainder nickel.

A surface treatment according to the invention may be carried out usingother powdered alloys provided that these contain not less than 6% byWeight of aluminiumand not less than 15% by weight of titanium, but itis to be noted that these elements must be alloyed in the powder whichis used. That is to say, the powder must be powdered alloy and not amere admixture of metal powders.

By way of example of the improvement in service life of an articleeffected by a surface treatment of the article in accordance with thepresent invention, the following is given:

A number of articles made from Nimonic 90 'were heated in contact withpowdered ferro-titanium hardener having the composition set forth aboveand in an atmosphere of commercially pure hydrogen from which all, orsubstantially all, traces of oxygen and moisture had been removed, forone hour at a temperature of 1100 C. The

Endurance limit on a basis ofl0 7 cycles (83 10 cycles (833 hrs.) hrs.)

Before treatment. After treatment 6.6 tons/ink".

:l: 1 :1: 14 tons/in. i 16.3 tons/in.

.2 :1: 15.4 tons/in.

These results show that the treated material is maintaining a higherlevel of fatigue resistance with increasing time and supportmetallographic tests upon used specimens of the treated article whichshowed no evidence of intergranular oxide penetration. Oxidation tests,operating by noting increase of weight with time, also showed that therate of oxidation has been reduced to about one half of that ofuntreated specimens.

In illustration of the improved resistance to thermal shock of articlestreated according to the invention, a specimen article of standardisednarrow angle wedge shape made of Nimonic and treated as above describedwas subjected to a thermal shock test in which a localised portion ofthe edge of the article was alternately heated in a gas flame and cooledin an air blast, the cycle being of one minute duration. This specimenwas found to survive 229 cycles. A similar article, surface treated inaccordance with the invention in the manner above described, but usingpowdered nickel-titanium hardener instead of ferro-titanium hardener,gave a life of 230 cycles following the same thermal shock test. Indistinction, however, three specimens made of Nimonic 90 fully agedafter machining, but not subjected to a surface treatment in accordancewith the invention, survived respectively without cracking only 70, andcycles following the above described thermal shock test.

Nimonic type refers to an alloy that is predominantly nickel andchromium. The specific example referred to as Nimonic 90 contains 18% to21% chromium, 15% to 21% cobalt and small percentages of titanium,aluminum, silicon, iron and manganese, the balance being nickel.

We claim:

1. A surface treatment for an article having a finished surface and madefrom a metal or alloy which is subject to oxidation penetrating betweenthe grain boundaries, said treatment comprising heating the article incontact with'a powdered alloy containing not less than 15 by weight oftitanium and not less than 6% by weight of 1 aluminium the remainderconsisting essentially of a metal pure hydrogen from which all, orsubstantially all, traces of oxygen and moisture have been removed.

3. A surface'treatment as claimed in claim 1, wherein the article iscooled in said inert atmosphere whilst still in contact with saidpowdered alloy.

4. A surface treatment as claimed in claim 1, wherein thearticle isheated in contact with powdered ferro-titanium hardener. a

5. A surface treatment as claimed in claim 1, wherein the article isheated in contact with nickel-titanium hardener.

6. A surface treatment as claimed in claim 1, wherein said powderedalloy has a fineness of between 100 and 16 mesh per inch.

7. A surface treatment as claimed in claim 1, wherein the article isheated in contact with said powdered alloy and in said inert atmospherefor a time sufiicient to form on the article a modified layer of athickness from 0.0005 inch to 0.001 inch.

8. A surface treatment as claimed in claim 1, for an article made of anickel base alloy containing chromium and cobalt, wherein the article isheated in contact with said powdered alloy and in said inert atmosphereat a temperature substantially between 1000 C. and 1190 C.

9. A surface treatment as claimed in claim 8, wherein the article isaged by reheating the article between 690 C. and 710 C. for a period oftime between twelve hours and sixteen hours.

Rossi Mar. 14, 1911 Ramage June 5, 1951

1. A SURFACE TREATMENT FOR AN ARTICLE HAVING A FINISHED SURFACE AND MADEFROM A METAL OF ALLOY WHICH SUBJECT TO OXIDATION PENETRATING BETWEEN THEGRAIN BOUNDARIES, SAID TREATMENT COMPRISING HEATING THE ARTICLE INCONTACT WITH A POWDERED ALLOY CONTAINING NOT LESS THAN 15% BY WEIGHT OFTITANIUM AND NOT LESS THAN 6% BY WEIGHT OF ALUMINUM THE REMAINDERCONSISTING ESSENTIALLY OF A METAL SELECTED FROM THE GROUP CONSISTING OFNICKEL AND IRON AND IN AN ATMOSPHERE WHICH IS INERT IN RELATION TO THEMETALS OF WHICH THE ARTICLE AND THE POWDERED ALLOY ARE COMPOSED, THEARTICLE BEING HEATED AT A TEMPERATURE SUBSTANTIALLY BETWEEN 1000*C. AND1190*C., AND FOR A TIME SUFFICIENT TO FORM A MODIFIED LAYER ON THESURFACE OF THE ARTICLE, THE ARTICLE BEING THEREAFTER COOLED.